A pair of conductive lines are coupled when they are spaced apart, but spaced closely enough together for energy flowing in one to be induced in the other. The amount of energy flowing between the lines is related to the dielectric medium the conductors are in and the spacing between the lines.
Couplers are electromagnetic devices formed to take advantage of coupled lines, and may have four ports, one for each end of two coupled lines. A main line has an input end connected directly or indirectly to an input port. The other end is connected to the direct port. The other or auxiliary line extends between a coupled port and an isolated port. A coupler may be reversed, in which case the isolated port may become the input port and the input port may become the isolated port. Similarly, the coupled port and direct port may have reversed designations. Couplers may be used as power combiners or splitters (dividers).
Directional couplers are four-port networks that may be simultaneously impedance matched at all ports. Power may flow from one or the other input port to the pair of output ports, and if the output ports are properly terminated, the ports of the input pair are isolated.
The Lange coupler is a four-port, interdigitated structure developed by Dr. Julius Lange around 1969. The length of the interdigitated fingers may be about one-quarter of the wavelength of a design frequency.